A large number of human genetic disorders could be treated by expression of missing or mutant genes in the liver. These disorders include familial hypercholesterolemia (deficiency of LDL receptors), ornithine transcarbamylase deficiency (a lethal liver metabolic disease), and hepatobiliary disease of cystitic fibrosis to name but a few metabolic disorders which effect the liver. In addition to correction of metabolic disorders effecting the liver, a number of primary tumors of the liver are known and would benefit from expression of anti-neoplastic genes in the liver [e.g., VDEPT; Huber et al. (1991) Proc. Natl. Acad. Sci. USA 88:8039].
In addition to permitting correction of inherited disorders which effect the liver, the ability to express genes in the liver permits gene therapy for a number of disorders whose primary defect is not located in the liver. For example, a number of inborn errors of metabolism result in high concentrations of toxic metabolites in the blood; transfer of a correct gene encoding the defective enzyme to the liver could permit metabolism of the toxic metabolites relieving the metabolic defect even though the site of the deficiency is outside of the liver (e.g., replacement of adenosine deaminase to remove toxic levels of adenosine and deoxyadenosine in the circulation of severe combined immunodeficiency patients).
Current approaches to targeting genes to the liver have focused upon ex vivo gene therapy. Ex vivo liver-directed gene therapy involves the surgical removal of liver cells, transduction of the liver cells in vitro (e.g., infection of the explanted cells with recombinant retroviral vectors) followed by injection of the genetically modified liver cells into the liver or spleen of the patient. A serious drawback for ex vivo gene therapy of the liver is the fact that hepatocyctes (i.e., liver cells) cannot be maintained and expanded in culture. Therefore, the success of ex vivo liver-directed gene therapy depends upon the ability to efficiently and stably engraft the genetically modified (i.e., transduced) hepatocyctes and their progeny. It has been reported that even under optimal conditions, autologous modified liver cells injected into the liver or spleen which engraft represent only a small percentage (less than 10%) of the total number of cells in the liver [Chowdhury et al. (1991) Science 254:1802]. Ectopic engraftment of transduced primary hepatocytes into the peritoneal cavity has been tried to address the problem of engraftment in the liver [Ledley, et al. (1987) Proc. Natl. Acad. Sci. USA 84:5335; Wilson et al. (1988) Proc. Natl. Acad. Sci. USA 85:3014 and Wolff et al. (1987) Proc. Natl. Acad. Sci. USA 84:3344].
Given the problems associated with ex vivo liver-directed gene therapy, in vivo approaches have been investigated for the transfer of genes into hepatocytes, including the use of recombinant retroviruses, recombinant adenoviruses, liposomes and molecular conjugates [Jaffe et al. (1992) Nature Gent. 1:372; Kaneda et al. (1989) Science 243:375; and Wu et al. (1989) J. Biol. Chem. 16985]. While these in vivo approaches do not suffer from the drawbacks associated with ex vivo liver-directed gene therapy, they do not provide a means to specifically target hepatocytes. In addition, several of these approaches require that a partial hepatectomy be performed in order to achieve prolonged expression of the transferred genes in vivo [Wilson (1992) J. Biol. Chem. 267:963].
Ideally, liver-directed gene therapy would be achieved by in vivo transfer of genes using vectors which specifically target hepatocytes. Hepatotrophic viruses, such as human hepatitis B virus (HBV), can be delivered via the circulation and their gene products are known to be expressed specifically in the liver. However, to date, the ability to express a foreign gene in the context of a HBV has not been reported. The art needs human HBV vectors capable of carrying and expressing foreign genes to allow in vivo liver-directed and liver-specific gene therapy.